The increasing world-wide demand for energy is accelerating fossil fuel consumption, depleting natural resources, and contributing to climate change (USEIA, 2011). With roughly 80% of the world's primary energy supply derived from fossil fuels, there is significant interest in increasing the contribution of renewable fuels to the overall energy production portfolio. Liquid fuels generated from lignocellulosic biomass are of particular interest as transportation fuels for long-term environmental and economic sustainability.
The Energy Independence and Security Act created a roadmap for increased industrial production of biofuels from cellulosic biomass in the United States (Public Law 110-140, 2007). According to the roadmap, the production of renewable fuels from cellulosic biomass was expected to reach 1.75 billion gallons by 2014 (Public Law 110-140, 2007). The actual production was only 683,643 gallons, (U.S. Environmental Protection Agency RFS2 Data, 2014) and the first generation of commercial-scale biorefineries in the U.S., to be in full operation in 2015, will not exceed an annual capacity of 50 million gallons (U.S. DOE, 2014).
Major bottlenecks still exist for the cost-effective production of biofuels from cellulosic biomass. Some of the challenges are economic and brought about by the massive amounts of fossil fuels that can now be tapped with horizontal drilling and hydraulic fracturing, which contribute to instability in the price of fossil fuels. Other challenges are technical, requiring new scientific and engineering innovation to bring transformational changes and cost reductions to the cellulosic biofuels industry.
One of the persistent challenges to implement cost-effective fermentation processes is the presence, in the hydrolysates derived from biomass, of plant-derived aromatic compounds and other small bioactive molecules produced during biomass deconstruction (Palmqvist et al. 2000, Piotrowski et al. 2014). Some of these molecules have been shown to diminish biofuel production by inhibiting growth and metabolism of sugars in fermenting organisms. For instance, acetic acid is known to affect cellular processes, reduce ethanol yields, and lower sugar consumption in wild type and engineered strains of Saccharomyces cerevisiae, (Bellissimi et al. 2009, Swinnen et al. 2014) whereas the negative effects of a variety of aromatic compounds on ethanologens such as S. cerevisiae, Zymomonas mobilis, and Escherichia coli are well documented (Chambel et al. 1999, Iwaki et al. 2013, Klinke et al. 2003, Delgenes et al. 1996, Zaldivar et al. 1999, Sato et al. 2014).
The suite of inhibitory molecules in hydrolysates is diverse (Piotrowski et al. 2014). Several strategies have been employed to overcome the effect of these inhibitory bioactive molecules (Larsson et al. 1999, Jonsson et al. 1998, Parawira et al. 2011). Although detoxification can be achieved by different approaches, in most cases the removal of the inhibitory compounds is accompanied by consumption of a significant amount of sugars (e.g., 5 to 35%) (Parawira et al. 2011).
There is a need to selectively remove inhibitory aromatic compounds from lignocellulosic biomass hydrolysates, lignin extracts, or other aromatic compound-containing solutions, without consuming the sugars needed for biofuel production. There is also a need of strategies to degrade or biotransform the large variety of plant-derived aromatics in lignocellulosic biomass hydrolysates, lignin extracts, or other aromatic compound-containing solutions into compounds that can be recovered and used for other applications.